1. Field of the Invention
The present invention relates to a projection screen for displaying a light image which is projected from, for example, a projector apparatus.
2. Description of the Related Art
A projector apparatus of this type for enlarging and projecting an image displayed on a cathode-ray tube (CRT), is known. However, such a projector apparatus requires the use of a high-luminance CRT, resulting in a very expensive and bulky apparatus.
A projector apparatus may be conceived which employs a liquid crystal display panel in place of a CRT type.
This type of projector commands the displaying of an image on a transmission type liquid crystal display, then enlarges and projects the displayed image onto a screen. Projectors falling under this category include a type which projects an image onto a separate external screen and a type which projects an image onto a transmission type screen situated within the projector body.
FIG. 1 shows a conventional LCD-type projector for projecting an image onto a transmission type screen situated within the projector body. In FIG. 1, reference numeral 1 denotes the outer case of the projector. Display window 2 having transmission type screen 3 is located on the front surface of case 1. Reference numeral 4 denotes a transmission type liquid crystal display panel arranged to face screen 3 inside case 1. Reference numeral 5 denotes a light source for illuminating panel 4 from behind. Reference numeral 6 denotes a projection lens for enlarging and projecting an image displayed on panel 4 onto screen 3. Projection lens 6 is made up of a plurality of optical lenses. Light source 5 comprises high-luminance light source lamp 5a, and reflector 5b (a reflector having a parabolic reflection surface, for reflecting light from light source lamp 5a as parallel light) for reflecting illumination light from lamp 5a toward panel 4. An ultraviolet component is removed from the light emitted from light source 5 by ultraviolet absorption filter 7a and ultraviolet reflection filter 7b. The filtered light is then incident on panel 4, through condenser lens 8. Condenser lens 8 corrects the light incident on panel 4, so that it is aligned parallel to optical axis O. When reflector 5b of light source 5 is a parabolic reflector, the light beams emitted from light source propagate toward panel 4, while at the same time being slightly diffused. Therefore, the light beams are corrected to be parallel beams by condenser lens 8, so that illumination light which is parallel to optical axis 0 can be incident on panel 4. Panel 4 comprises a dot-matrix display panel for displaying, for example, a television image. Optical image passing through panel 4, i.e., a display image on panel 4, is focused on projection lens 6 by circular Fresnel lens 9, and is enlarged and projected onto screen 3 of display window 2 by projection lens 6. FIG. 1 shows an LCD-type projector using a parabolic reflector as reflector 5b of light source 5. In a liquid crystal projector using an elliptic mirror surface reflector as the reflector, a relay lens is arranged between the light source and the liquid crystal display panel, so that light reflected by the reflector is corrected by the relay lens to be parallel light and is guided to the liquid crystal display panel.
In this liquid crystal projector, the display image on panel 4 is enlarged and projected onto screen 3 on the front surface of the projector body, and is watched from the front surface side of the projector body. With this projector, the display image on a small liquid crystal display panel can be enlarged and watched. In this liquid crystal projector, since screen 3 is provided to the projector body, the projection screen does not have a large size relative to a liquid crystal projector for projecting an image onto an external screen. However, since the external screen is not necessary, the above projector shown in FIG. 1 can be used in any location.
The screen 3 is of a transmission type and is used as a rear-surface projection screen for a projection apparatus, such as a rear-surface projection type projector, which enables a light image which is projected from behind the screen to be observed on the major surface of the screen. This type of screen is formed of a lenticular lens having a greater number of lens units in a continuous array on an observation surface, that is, a major surface of the screen.
The rear-surface projection screen having the lenticular lens allows light which is incident on the rear surface of the screen to emerge on the major surface of the screen in a diffused fashion. Such a screen has the advantage of providing a projection image at a greater field of vision than a planar screen.
Known is a screen constructed of a lenticular lens having lens units formed in a continuous array on the major surface with a top surface formed as a lens surface and with a pair of side-inclined surfaces formed, as a pair of total-reflection surfaces relative to the top surface to allow light which is incident on the rear surface of the screen to be directed to the lens surface. Since the conventional rear-surface projection screen has a pair of straight-inclined surfaces as the side-inclined surfaces at each of the lens units over the whole surface of the lenticular lens, it follows that, of light which is reflected by the total-reflection surface toward the lens surface of the lens unit, some which is incident on the lens surface at a near total-reflection angle is reflected back from the lens surface in the rear-surface direction of the screen. As a result, there occurs a great loss of light so that the image screen appears dark.